Softened butter and sugar, for the ramekins
8 ounces semisweet or bittersweet chocolate, finely chopped
8 tbsp. (1 stick) unsalted butter, cut into bits
2 tbsp. milk
3 tbsp. unsweetened cocoa powder
2 large eggs, separated, at room temperature
1 large egg white, at room temperature
1/8 tsp. cream of tartar
3 tbsp. sugar
Sweetened whipped cream (to garnish)
1. Lightly butter insides of six 6-ounce custard cups, sprinkle with sugar; tap out the excess.
2. In a medium stainless steel bowl placed in a skillet of hot, not simmering, water, melt chocolate and butter, stirring often, until smooth. Remove from heat. Measure 6 tbsp. into a small shallow bowl. Add milk and stir well. Place in freezer while preparing rest of recipe. Stir cocoa and egg yolks into remaining chocolate-butter mixture.
3. In a grease-free medium bowl, whip egg whites with cream of tartar until soft peaks form. Gradually beat in sugar until peaks are stiff and glossy but not dry. Fold about 1/4 of egg whites into cocoa mixture, then fold in remaining whites. Spoon batter into custard cups, filling half full. Set remaining batter aside.
4. Remove chocolate-butter mixture from freezer; it should be firm. Using a dessert spoon, scrape up mixture and roll into 6 truffle-like balls. Embed a truffle into the center of each custard cup, top with remaining batter and level the tops. Place on a baking sheet. Cover with plastic wrap and refrigerate at least 3 hours. (Can be prepared and refrigerated up to 3 days ahead.)
5. Position rack in lower third of oven; preheat to 400 F. Bake until cakes are puffed and cracked and a toothpick inserted in the center comes out gooey, indicating the truffle is melted, about 12 minutes. Let cool for 3 minutes.
6. Run a thin knife around inside of each cup to release the cake. Protecting your hands with a towel, invert a cup in the center of a dessert plate to unmold. Repeat with remaining cakes. Serve immediately with a dollop of sweetened whipped cream.